It has come to be recognized that global warming caused by an increase in CO2 concentration driven by the use of a large amount of fossil fuels and an increase in energy demands driven by population growth have posed problems of annihilation of the human species. In recent years, therefore, studies are being energetically made for the utilization of sunlight that is infinite and free from the occurrence of harmful substances. For utilizing the above sunlight that is a clean energy source, there are practically used inorganic solar cells for residential buildings, such as a solar cell of single crystal silicon, polycrystal silicon, amorphous silicon, cadmium telluride and indium copper selenide.
However, silicon for use in the above solar cell is required to have very high purity, and the purification step thereof is complicated and requires a large number of processes. The solar cell requires a high production cost. The solar energy generation based on the above inorganic materials is disadvantageous in view of a cost and a long period of redemption for users, which have been problems that hinder the spread thereof.
On the other hand, many types of solar cells using organic materials have been also proposed. The organic solar cells include a Schottky type photoelectric conversion device having a junction formed by a p-type organic semiconductor and a metal having a small work function and a hetero-junction type photoelectric conversion device having a junction formed by a p-type organic semiconductor and an n-type inorganic semiconductor or a junction formed by a p-type organic semiconductor and an electron-accepting organic compound. The organic semiconductor used contains a material selected from synthetic dyes or pigments such as chlorophyll or perylene, electrically conductive polymer materials such as polyacetylene or composite materials of these. A thin film is formed from any one of these materials by a vacuum vapor deposition method, a casting method, a dipping method, or the like to constitute a cell material. While the organic materials have advantages that they are less expensive and permit the easy formation of a large area, they have problems that many of them exhibit a conversion of 1% or less and that they are poor in durability.
Under the circumstances, “Photoelectric conversion device using dye-sensitized semiconductor fine particles and solar cells” reported in Nature (Vol. 353, page 737, 1991) and U.S. Pat. No. 4,927,721 was remarkable. The above documents also disclose materials and a production technique, which are necessary for producing the cell. The proposed cells are called “Graeztel” type, and they are wet solar cells using, as a work electrode, a porous thin film of titanium oxide spectrally sensitized with a ruthenium complex. The above method has the following advantages; It is not required to purify a semiconductor of a less expensive oxide such as titanium oxide until it has a high purity, so that the cells are less expensive, and light that can be utilized covers up to a broad visible light region, so that sunlight containing a large quantity of visible light components can be effectively converted to electricity.
On the other hand, the above cells use a very expensive ruthenium complex and require an improvement in view of a cost. This problem can be overcome if the expensive ruthenium complex can be replaced with a less expensive organic dye such as cyanine or the like. A cyanine dye and a merocyanine dye have been developed as a dye for the above cells (JP-A-11-238905, JP-A-2001-52766 and JP-A-2001-76773). However, these dyes have low adsorption to titanium oxide or cannot yet produce an high sensitization effect, and they also have a problem with regard to stability with the passage of time (durability).
Under the circumstances, it is a first object of the present invention to provide a novel organic dye that has excellent stability with the passage of time and an excellent photoelectric conversion property and which is suitable for use in a semiconductor electrode, or the like, and a photoelectric conversion material containing the dye. It is also a second object of the present invention to provide a semiconductor electrode to which the above organic dye is applied and a photoelectric conversion device excellent in photoelectric conversion efficiency.